1. The Field of the Invention
The present invention relates in general to the improvement of the fracture strength or tensile strength of end sections of glass tubes against mechanical or thermal loads, as they typically occur in the course of further processing at the respective end section, and relates in particular to a process for the production of glass tubes with reduced stress in or at at least one tube end, preferably in or at both tube ends, wherein according to a further aspect of the present invention the intention is to create a predetermined stress profile in at least one tube end section.
2. The Description of the Related Art
After being cut or cut to length from a glass tube stream, industrially produced glass tubes have a specific stress curve, such as is illustrated by way of example in FIG. 1a, in the region of the tube ends and after heating of the tube ends. This stress curve is the result of the cutting process used and is induced by subsequent fusing of the tube ends. The tube end described here lies by way of example in the region at a distance between 0 and 50 mm to the respective tube end.
If such a tube end is used directly and without appropriate post-treatment in a glass-to-metal joint, the result due to the steep gradient between tensile stress and compressive stress at the tube end after thermal processing, e.g. on heating with glass solder, is splitting off during the cooling process due to stress induction (also referred to as “ring fracturing” of the tube end.) This leads to the complete destruction of a glass-to-metal joint such as is illustrated by way of example in FIG. 3b and will be described in greater detail below.
In order nevertheless to use glass tubes for glass-to-metal joints the entire glass tube is subjected according to the prior art to what is referred to as precision cooling. For this purpose the entire glass tube passes, after being severed from the glass tube line and cooling to ambient temperature, into a separate oven in which the entire glass tube is subjected to a controlled heat treatment, i.e. a predetermined heat treatment with precisely defined heating and cooling curve. FIG. 1b illustrates the stress curve at a tube end of such a glass tube that is conventionally subjected to precision cooling. In the region of the arrows, as indicated by reference letters A and B, there is a significant reduction in the compressive stress (in region A) and tensile stress (in region B) as a result of the precision cooling.
There are cooling curves adapted to each type of glass that are supposed to bring about a reduction in the stress. The general doctrine says that for the stress relief of glass a time period of at least 15 min is required between the annealing point at a viscosity η=1013.5 Pa s and the strain point at a viscosity η=1012 Pa s to reduce the stress to such an extent that it is possible to process the glass safely (without breakage). Subsequently, the temperature of the glass item must be decreased at a rate of 2° C./min from the annealing point to the strain point. Such a precision cooling of glass tubes is therefore very time-consuming because several hours are usually needed for heating up and cooling down.
Such a precision cooling is also very costly and usually consumes a lot of energy. An annealing lehr is normally used for cooling glass or for controlling the stresses from the center of the tube towards the end of the tube. For this purpose a glass item or the glass tube is placed into the annealing lehr, passes through a predetermined cooling curve and is then removed from the lehr and packaged.
In addition, the atmosphere in conventional ovens used for precision cooling is relatively dirty with the result that the glass tubes have to be laboriously cleaned after the precision cooling, for instance if they have to be coated for a subsequent use.
One way out in order to eliminate these problems is to cut off and discard the respective tube end sections after cooling down completely to ambient temperature. However, this reduces efficiency and increases the costs of glass tube production.
U.S. Pat. No. 2,166,871 A describes a process in which a glass gob is continuously severed, in a heat-softened state, from a glass tube line (glass drawing line) and subsequently formed. Viscosities or temperatures around the working point at a viscosity η=104 Pa s are required for this purpose. The glass gob thus produced is subsequently subjected to a heat treatment that is not described in greater detail.
GB 826,270A discloses a process for severing glass tubes that is primarily used for large glass tube diameters. The glass tube, after local heating using burners, is heated until melted off by applying a very high electric voltage. Due to this very rapid local overheating, the severed glass tube must be heated up immediately after being severed to prevent stress cracks (“runners”) induced by the high temperature difference. For this purpose the still heated glass tube is placed on a transport means which moves the glass tube past heating devices provided laterally. However, in this case there is no passage through a heating region with a predetermined temperature profile, which extends in the longitudinal direction of the glass tube and in which the temperature of the glass tube is selectively and locally increased depending on a distance to the respective tube end.